In the systems of water pipes included in nuclear power plants and serving to conduct water to and from, inter alia, the reactor and the condenser, there are a plurality of points at which water having a certain temperature is to be mixed with water having a different temperature. This took previously place in simple T-piece connections or branch pipe points at which an open branch pipe opens directly into an aperture in the circumferential wall of a main pipe. At such branch points, the two water flows meet in an uncontrolled manner during rather intensive vorticity which, inter alia, implies that vortices or streaks of water having a certain, e.g. higher temperature than other streaks of water move back and forth both axially and sideways along the inside of the pipe wall of the main in the area downstream of the branch point. This means that at least the inside of the main pipe is subjected to intermittently repeated variations in temperature, leading to the pipe material, which in practice in most cases is acid-proof steel, alternately being subjected to compressive and tensile stress. This phenomenon, so-called thermal fatigue, shows itself in crack formations in the pipe material. If the differences in temperature between the two intermixed fluids are great, for example 50.degree. C. or more, and the fatigue continues for a long time, the crack formation may advance so far as to jeopardise security. The inclination to form cracks will be especially pronounced in the area of welds which are frequently to be found in the vicinity of the branch point downstream thereof.
For the purpose of at least reducing the above-mentioned problems, attempts have recently been made to mount in the branch point between main and secondary pipes a special mixing device serving to control the mixing process in such a manner that the number of variations in temperature per unit of time along the internal surfaces of the pipe walls is reduced. For such mixing, use has been made of a connecting branch which extends essentially radially into the main pipe from the secondary pipe and in whose cylindrical circumferential surface there are formed a plurality of small perforations through which the water from the secondary pipe flows radially outwards in the form of a corresponding number of jets. In one embodiment, the connecting branch has been formed with perforations of the same size. In other embodiments, experiments have been made with apertures of different size. For example, the perforations of the connecting branch in the area of the main pipe centre have been made larger than the apertures closer to the peripheral wall of the pipe. These experiments have, however, not proved successful in so far as pronounced fluctuations in temperature along the pipe wall surfaces could not be prevented. Especially in variations of the water flows in the two pipes, the force of the jets through the perforations has increased and decreased and, since it was not be possible to prevent individual jets from hitting the inside of the main pipe, the jets will migrate along the surface of the pipe wall and cause variations in temperature in the pipe wall material.